1. Field of the Invention
The present invention relates to differential drive mechanisms, particularly automotive differentials of limited slip type, and is concerned with that type of differential drive mechanism which comprises a cage, which is rotatable about a first axis and represents the input, two coaxial output shafts which are rotatable with respect to the cage about the first axis, a coupling which is connected eccentrically to the two output shafts to transmit relative contra-rotational movement between them by connections which permit relative rotation of the coupling and the output shafts about a second axis substantially parallel to the first axis and a restraint member which is coupled to the cage and to the coupling such that the coupling is rotatable with respect to the cage about a third axis substantially perpendicular to the first axis and capable of reciprocating movement in a direction perpendicular to the first axis but prevented from movement in a direction parallel to the first axis, the eccentric connection of the coupling and the output shafts being constituted by a respective eccentric hole in the inner end of each output shaft in which the associated end of the coupling is received.
2. Description of the Prior Art
Conventional automotive differentials provide a cost effective means of sharing torque equally between the driven wheels. However, the characteristic of always providing equal torque restricts the total to the level which maintains traction at the wheels with minimum grip. If this is close to zero, e.g. due to the fact that one wheel is on very slippery ground, then the total will also be very low even if one wheel has very good grip.
To overcome this disadvantage, various types of limited slip differential have been used of either the speed control or torque control type. The speed control type senses wheel slip after it occurs by the speed difference and transfers torque to the slower wheel accordingly. The torque control type transfers torque from the faster moving wheel before traction is lost.
Various types of torque control automotive differential are known but substantially all of those which are actually used comprise a substantial number of meshing helical gears or alternatively face cams and cam followers with helical sliding surfaces. Such differentials are therefore relatively heavy and expensive and time-consuming to manufacture and are relatively complex. In addition, the areas of the friction surfaces at which friction torque is generated are relatively small which means that the loading per unit area is relatively high and thus that the service life is relatively short.
Numerous differentials of the specific type with which the present invention is concerned and of a generally similar type are also known from the patent literature, e.g. U.S. Pat. Nos. 1,098,422, 1,098,423, 1,364,745, 1,437,510, 1,954,347, 4,155,274, 4,291,591, 2,016,849, 848,931, 1,278,231, 1,854,910, 1,499,480, 1,463,356, 1,663,882 and 1,843,163. However, it is believed that no such differential has ever been manufactured as a commercial product. The reason for this is not known with certainty, but it is believed that, in addition to the fact that many of the differentials disclosed in the patents referred to above are of very complex construction and thus prohibitively expensive to manufacture, they all suffer from the following shortcoming: The entire driveline torque of an automotive engine is normally transmitted through the differential at all times, that is to say through those components whose primary purpose is to permit differential rotational speeds of the two output shafts. In this connection it will be appreciated that the loads to which the differential components are subjected when the vehicle is travelling in a straight line, for instance shock loads, particularly under heavy acceleration, are very considerably greater than the additional loads which are produced when the output shafts rotate at different speeds, particularly because the speed differential is rarely more than a few r.p.m. This means that the components in question must be of very robust and thus expensive construction and furthermore are subjected to very substantial loads, point or line loads in some cases, for extended periods of time and are thus subject to unacceptably premature failure through breakage or wear.
German Patent No. 819628 discloses a differential of the type referred to above which suffers from all the disadvantages referred to above. Thus all the driveline torque of the engine is transmitted through the differential components, i.e. the coupling, the restraint member and the female eccentrics in the output shafts, which means that these components must be of very robust construction but are nevertheless prone to unpredictable premature failure. Furthermore, the substantial loads exerted by the ends of the coupling on the walls of the eccentric holes fall outside the support of the bearing into the cage and result in substantial wear of both these components and in bending of the output shafts which in turn results in excessive wear and premature failure of their bearings.
U.S. Pat. No. 1,954,347 discloses a differential of the type referred to which superficially appears to be of some relevance to the present invention in that the outer surfaces of the output shafts are in close proximity to the inner surface of the cage. However, they are said to be bearing fits within the cage or housing and this makes it clear that it is the intention to minimise contact and friction between the opposed surfaces and this is in stark contrast to the construction of the present invention which relies on frictional sliding contact between the cage and the output shafts to produce the split in the torque path through the differential. The distinction is further made clear by the fact that there is no split in the torque path in the prior patent.
Furthermore, the prior patent makes no provision for containment of the loads and wear regimes that will be experienced, in use, by the differential. As such, the construction of the prior patent suffers from a number of severe disadvantages which, it is believed, were responsible for it never being manufactured as a commercial product. Firstly, with respect to the generation of the load across the sliding interfaces between the coupling and the output shaft, this load is inversely related to the eccentricity of the coupling surface. The eccentricity is necessarily small in the prior patent and any increase in the eccentricity would result in an increase in the overall diameter and thus an unacceptable increase in the size of the differential. Secondly, the interface between the coupling and the output shaft is a line contact only which would cause unacceptably high Hertzian contact stresses and excessive and premature wear. This problem is solved in the present invention by the use of the slippers but this would not be possible in the prior construction since it would again result in an excessively large overall diameter. Thirdly, if the interface of the outer surface of the inner ends of the output shafts and the cage were to support the high loads generated by the small eccentricity necessary in the prior construction, serious problems would arise because no provision is made for the necessary abrasion resistance.
It is the object of the invention to provide a differential drive mechanism of the general type referred to above whose components are required to transmit only a proportion of the normal propulsive power of the vehicle engine or the like and are thus subjected to lower loads than usual and in which the high internal loads are simply supported on frictional surfaces, whereby they may be of lighter and cheaper construction than usual and the differential has a longer service life than known differentials.